After a fresh install running admin/check.php gives WARN for check:
core_path configuration option is set to a path outside the web root
For increased security it is recommended that you move the core_path directory outside the web root.

Moving the directory outside the web root does not work as there is a hardcoded path in core.php
require_once( dirname( FILE ) . DIRECTORY_SEPARATOR . 'core' . DIRECTORY_SEPARATOR . 'constant_inc.php' );

constant_inc.php has been moved to core folder in 2003, see commit 5cad7a7e23fba2a51a369a764daf33aeee232ddd
At first sight it seems that $g_core_path can't be changed since that time.

Activities

This problem occurs because when core.php is loaded the config file has not been read yet. So if you set $g_core_path in /config/config_inc.php the variable is still not defined at that moment.

A work around is to move the whole core directory to an outside path. Then recreate the core folder and copy the file constant_inc.php to that newly created core folder from the outside path core folder. This will result in an empty core folder that only contains constant_inc.php.

You can also move the config folder to the outside location. And then you have to still keep the config folder and /config/config_inc.php. But you can edit that new config and change its content to only the paths and the include_once( $g_config_path . 'config_inc.php' ).

I tried this with 2.11.1 but when I tried to go to the login page, I get a blank. only when I put the config back in the mantisbt root directory does this work. This also happens when i move the core as well. Please advise as to what I could be doing wrong.

It extracts the path to your "config" folder from the environment variable of your web server.
Add the following line to your web server's configuration file:
SetEnv MANTIS_CONFIG_FOLDER /path to your config folder/